In recent years, the amount of communication has been ever-increasing due to popularization of broadband networks, such as the Internet, and establishment of a large-capacity photonic network has been demanded accordingly. Under the present circumstances, a network at a communication speed of 10 Gbps is in the mainstream. Development of transmitters including an optical modulator and receivers adaptable to a communication speed to 40 Gbps has been demanded to cope with a further increase in the amount of communication. In order to modulate an optical signal of 40 Gbps, an optical modulator of LiNbO3 (LN modulator) is used, for example. Driving the LN modulator at 40 Gbps requires an amplifier circuit capable of a high-speed and large-amplitude modulated output.
FIG. 7 illustrates a two-stage feedback amplifier circuit according to a related art. In FIG. 7, the two-stage feedback amplifier circuit 70 includes an amplifier circuit of a two-stage configuration, in which an output of a second-stage amplifier is fed back to a bias of a first-stage amplifier. That is, electric signals including modulation information input to “IN” and “IN” (negative logic) are amplified by corresponding two stages of transistors T1 and T2 and T3 and T4, respectively, and are output from output terminals “OUT”. The output of the transistor T2 is fed back to the bias of the transistor T1, which is a first-stage amplifier, whereas the output of the transistor T4 is fed back to the bias of the transistor T3, which is a first-stage amplifier. Accordingly, an amplification gain of a high-frequency component increases.
That is, a feedback current from the output of the transistor T2 flows through resistors R2 and R3 and is fed back to the bias of the transistor T1. Likewise, a feedback current from the output of the transistor T4 flows through resistors R5 and R6 and is fed back to the bias of the transistor T3. The configuration enables an increase in amplification gain of a high-frequency component and a broader amplification band.
FIG. 8 illustrates a configuration of a two-stage feedback amplifier circuit according to a related art. In the two-stage feedback amplifier circuit 80 illustrated in FIG. 8, source follower transistors T5 and T6 are connected between resistors R2 and R3 and between resistors R5 and R6, respectively. A feedback current from an output of a transistor T2 is supplied to a bias of a transistor T1, which is a first-stage amplifier, via the transistor T5. A feedback current from an output of a transistor T4 is supplied to a bias of a transistor T3, which is a first-stage amplifier, via the transistor T6.
The following configuration of a multistage high-frequency power amplifier circuit including a plurality of cascaded power amplifying transistors has been known and disclosed in Japanese Unexamined Patent Application Publication No. 2004-193846, for example. That is, in the configuration, distortion of a signal is reduced by allowing a sufficient idle current to flow to an amplifying transistor before the last stage even in a region of a low output power level, thereby enhancing power efficiency.
Also, a related art of an amplifier circuit to suppress an influence of feedback capacitance and to realize a broad band at low cost is described in Japanese Unexamined Patent Application Publication No. 08-256024. Also, there is known an amplifier circuit including total-feedback amplifier circuits and level-shift amplifier circuits alternately connected in multi-stages. In the amplifier circuit, a capacitance component provided between transistors of the level-shift amplifier circuit suppresses emitter negative feedback resistance in high frequencies, increases the gain of high frequencies, and realizes a broader bandwidth. Such an amplifier circuit is disclosed in Japanese Unexamined Patent Application Publication No. 10-247831, for example.
In the case where the two-stage feedback amplifier circuit illustrated in FIG. 7 or 8 is used as a driver of an LN optical modulator of a high-speed optical communication apparatus on a transmission side, a signal level necessary to drive the optical modulator is obtained by adjusting the amount of current flown to a current source I. That is, when the LN optical modulator is connected to the output terminals “OUT” and the amplifier circuit is used as a driver of the LN optical modulator, an output amplitude of the driver depends on the amount of current flowing in the current source I, and thus the output amplitude is adjusted by adjusting the amount of current, whereby an output level necessary to drive the optical modulator used is obtained.
However, if the amount of current flowing in the current source I is changed in the amplifier circuit illustrated in FIG. 7 or 8, the potentials at respective points A, B, and C illustrated in FIGS. 7 and 8 change at the same time. Particularly, the potential at point C corresponds to a gate potential of the transistor T2, which is a second-stage amplifier. An abnormal operation of the transistor T2 occurs due to a change in potential level at point C, and as a result an output waveform significantly degrades. This is the same in the gate potential on the transistor T4 side.